I think it is obvious on a fundamental level following professional sports is inane. I love baseball, and to a lesser degree other sports, but I acknowledge that my enjoyment and interest in these sports is compeletely irrational. On the other hand I don’t think sports are harmful in any way and as entertainment goes they are among the best. (definitely better than 95% of video games and movies).
I hope one day to be on a level where every action I take is l’shem shomayim and sports will be of no interest to me, but I am very far from that point and sports are way down on the list in terms of what I need to work on.
For someone who has never followed sports and first comes across the phenomenon in adulthood, it would be surprising if they didn’t mock it. I don’t think it is so terrible for a Rebbe or a Rav to occasionally remind his flock that professional sports are hevel, but without declaring them ossur or denigrating his students for enjoying them.